In the semiconductor industry there is a constant demand for thinner and thinner components, or wafers from which these components are isolated. In the production and in particular in the thinning of wafers, laminar structure solutions are employed, for protecting and mechanically stabilizing the wafer that is to be thinned during the thinning process. These layer systems also perform the stated functions in subsequent processing steps. In particular, however, they serve to stabilize thinned wafers, which owing to their small thickness are particularly mechanically sensitive. Layer systems are used for this, comprising materials that are the most suitable for the particular purpose, e.g. films, other wafers or glass plates. These parts of the layer systems can in their turn be combined with wax, elastomers or other plastics.
If, in these layer systems, a glass plate or a disk with mechanical properties comparable to the wafer that is to be thinned—as naturally in particular another wafer possesses—is used as carrier, which in particular is to serve for stabilizing the complete layer system, it is advantageous and in some circumstances even necessary for a layer to be present between the surface to be protected and this carrier, said layer joining together the wafer to be processed and the carrier. This layer must on the one hand ensure sufficient adhesion between the carrier and the wafer to be processed, and on the other hand it must be able to even out the topographic irregularities that are usually present on the surface of the wafer to be processed. These topographic irregularities are caused for example by electronic components on the active side of the wafer, which of course must be protected during thinning, as well as for example by contacts such as bumps. The bonding layer can of course also be a layer system, comprising several different layers, which advantageously are complementary with respect to their properties, e.g. adhesion promoting, elasticity, hardness, thermal conductivity, chemical resistance etc. in the sense of the intended application.
In the production of very thin wafers, thinning imposes a high mechanical loading on the wafer that is to be thinned. As the wafer that is to be thinned has often already gone through a large number of manufacturing steps and in particular often already includes the electronic components on its active side (front), also from the economic standpoint it is extremely important that breakage of the wafer and therefore rejection are avoided as far as possible during thinning. For this purpose, before high mechanical loading occurs, while the wafer is still in the unthinned state it is bonded to the carrier. The back that is to be thinned (i.e. the side that does not comprise the electronic components) must of course remain free. Joining of the wafer to its carrier is also called bonding.
After bonding, the wafer is thinned on its back, as a result of which—as already mentioned—it loses mechanical stability. This is compensated by the carrier during thinning and subsequently. Accordingly, the carrier must provide a more stabilizing action, the thinner the wafer is at the end of the thinning process. Accordingly, in the case of relatively thick wafers as end product, films can also be used as carriers. Basically it is possible that, during thinning, the wafer is already separated into its individual components. In many cases this is desirable, as it makes a subsequent separating step unnecessary. Separating can for example be achieved by providing, on the active side of the wafer between the individual components (dice), depressions that are deep enough so that during the thinning of the backs, these depressions are already reached and therefore pass through.
The company DISCO HI-TEC offers a process, called “Dicing by Grinding”, in which the wafer can, by means of thinning, also be separated. In this, structures are ground-in, scribed or etched on the front of the wafer before applying the carrier. These structures have a depth that is greater than the final thickness to which the wafer is thinned. Accordingly, as a result of thinning, as described above, the structures are opened and the wafer is thus separated.
As a result of separation, the overall risk of breakage is certainly reduced, but the separated components must in their turn be protected from mechanical loading.
There is always the problem in thinning that, after thinning, the carrier has to be separated from the wafer. In particular, the carrier must be separated because it blocks access to the electronic components on the active side of the wafer. As a rule, removal of the carrier does not pose any problem in the case of relatively thick wafers, for two reasons:                The relatively thick (only slightly thinned) wafer can still withstand mechanical loading to a certain extent and        the greater the mechanical loading that the wafer can withstand in its final thickness, the more flexible the carrier can be. Accordingly, flexible carriers e.g. in the form of films can simply be pulled off mechanically.        
With carriers that are less flexible, e.g. glass plates or other wafers, pulling off in this way is not of course possible. In this case separation is particularly difficult, as hard carriers are used in particular with greatly thinned wafers, so that high mechanical loading must definitely be avoided.
For this purpose, often layers are used between the wafer and the carrier, which as a result of chemical or physical change, reduce or eliminate the force of adhesion between the wafer and the carrier. An example of such a layer is wax, which becomes soft under the action of heat and thus facilitates separation. For this, the wax is heated until it is sufficiently fluid for the carrier to be displaced relative to the wafer. A disadvantage of this method is that in particular the sensitive wafer surface, which comprises the electronic components or the contacts to these, must be cleaned afterwards.
Basically, it is also possible to use special adhesives, which similarly lose their adhesiveness under the action of heat or radiation. Such a method is offered for example by the 3M company, where release of an adhesive layer is brought about by the use of laser energy.
Once again there is the problem that parts of the adhesive layer remain on the wafer surface, necessitating expensive cleaning.
As cleaning means additional expense, causes additional mechanical and often also chemical stressing of the thinned wafer and/or it is very difficult to ensure that all residues of adhesive are adequately removed, for many applications purely mechanical solutions are preferred, in which a separating layer is pulled mechanically off of the active side of the wafer, so that no residues remain.
A separating layer of this kind is disclosed in WO 2004/051708. This separating layer is in its turn used in the further development of the thinning process disclosed in WO 2007/099146. In this last-mentioned patent application, after the thinning of the wafer, a second carrier is provided on the (thinned) back, which is to support the separating process from the first carrier. Separation takes place between the separating layer and the active side of the wafer. In WO 2007/099146, a mechanical separating process is disclosed, in which the assembly of wafer and second carrier is led over a roll, so that the separating operation is effected mechanically. This second carrier is often a dicing film. With this method the second carrier must be flexible, as it must be able to follow a curvature. A problem with this method is that during separation, the stretchable carrier for its part can undergo deformation in addition to the curvature caused by the roll, so that the separating operation is not sufficiently controlled. In particular with very thin wafers, which are of course particularly mechanically sensitive, the method disclosed in WO 2007/099146, when using the deflecting roll, often leads to excessive forces perpendicular to the plane of the wafer, so that wafer breakage may occur, because the stretchable carrier does not fully follow the curvature of the roll surface. As a result, the separating front does not lie directly under the support of the roll, but is slightly displaced laterally. This leads to an increase in the vertical component of the force exerted on the wafer.